1. The Nature of Photovoltaic Cells
A photovoltaic cell, often referred to as a solar cell, is a semiconductor junction device which converts light energy into electrical energy. A typical photovoltaic cell is a layered structure comprising four principal layers: (1) an absorber-generator (2) a collector-converter (3) a transparent electrical contact and (4) an opaque electrical contact. When light is shone into the absorber-generator, the device generates a voltage differential between the two contacts and an electric current which increases as the intensity of the light increases.
The absorber-generator (commonly referred to as the "absorber") is a layer of semiconductor material which absorbs light photons and, as a consequence, generates minority carriers. Typically the absorber absorbs photons and ejects electrons thus creating pairs of negatively charged carriers (electrons) and positively charged carriers ("holes"). If the absorber is a p-type semiconductor, the electrons are minority carriers; if it is n-type, the holes are minority carriers. As minority carriers are readily annihilated in the absorber by recombination with the plentiful majority carriers, they must be transported to a region wherein they are majority carriers before they can be utilized to power an electrical circuit.
The collector-converter (the "collector") is a layer of material is electrical contact with the absorber wherein the majority carriers are of the same conductivity type as the minority carriers generated in the absorber. This layer "collects" minority carriers from the absorber and "converts" them into majority carriers. If the collector is an oppositely doped region of the same semiconductor as the absorber, the photovoltaic devide is a homojunction device. If the collector is comprised of a different semiconductor, the device is a heterojunction; and, if the collector is metal, the device is a Schottky barrier junction.
The transparent contact is made of an electrically conductive material which permits light to pass through to the absorber. It is typically either a continuous transparent sheet of conductive material or an open grid of opaque conductive material. If the transparent contact is on the same side of the photovoltaic device as the absorber, the device is referred to as being in the front wall configuration. If the transparent contact is on the opposite side, the device is said to be in the back wall configuration.
2. History of the Art
Although scientists have known about the photovoltaic effect for more than a century, it is only within the past twenty-five years that it could be considered a practical means for generating electricity in useful amounts. Prior to 1950, photovoltaic devices were limited in use to highly specialized applications, such as light metering, wherein conversion efficiency was immaterial and electrical current demand was minimal.
The advent of silicon junction technology in the 1950's permitted the development of high cost, high conversion efficiency silicon junction photovoltaic cells. Arrays of such devices have been used with considerable success in the space program where cost is of little significance. However, the cost of such devices as energy generators, typically exceeding $10,000 per kilowatt, is prohibitively high for terrestrial applications wherein they must compete against conventional generators. While much of this coat is due to the high quality control standards required for spacecraft components, a substantial portion is due to the high cost of preparing silicon crystals of the required purity and due to the inefficiencies of the batch processes by which such cells are fabricated.
Thin film photovoltaic cells possess many potential advantages over silicon cells in terrestrial applications. Photovoltaic cells employing thin films of polycrystalline materials such as a copper sulfide absorber and a cadmium sulfide converter offer substantial advantages for the development of continuous processing techniques, and they are flexible and light of weight. Consequently they offer much promise as cells which can be easily fabricated, transported and deployed.
One difficulty that arises in the use of thin film photovoltaic cells utilizing copper-bearing absorbers is a gradual degradation of their energy conversion characteristics. When deployed in the field as solar cells, such cells gradually lose their ability to collect and convert solar energy into electrical energy. Typical copper sulfide cells lose approximately 29 percent of their conversion efficiency within a period of about two years. Such degradation would present serious difficulties in large-scale solar cell deployments.